JPH09145928A - Optical attenuator - Google Patents

Abstract

(57) [Summary] (Modified) [PROBLEMS] To provide an optical attenuator that prevents deterioration of transmission characteristics of an optical signal due to the influence of mutual interference between core propagation mode light and cladding propagation mode light. SOLUTION: A front optical fiber 1 via an input end 101.
02, the high-refractive-index liquid 109 of the cladding-propagation-mode removing unit 108 installed in the front optical fiber 102 has a sufficiently higher refractive index than the cladding unit 106. In the material, the clad propagation mode light is transmitted to the high refractive index liquid 109 side without being totally reflected at the boundary surface thereof, and the clad propagation mode light does not reach the connection point 105. The clad propagation mode light of the rear optical fiber 104 is also transmitted to the high refractive index liquid 109 side of the clad propagation mode removing part 108, and the clad propagation mode light is attenuated and does not reach the light emitting end 103.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical attenuator,
More specifically, it has a fixed amount of attenuation,
The present invention relates to an optical attenuator that attenuates an optical signal without newly generating noise.

[0002]

2. Description of the Related Art Conventionally, an excessive optical signal is prevented from being incident on a light receiving element or an optical component installed in an optical transmission system, or an optimum optical signal is incident on the light receiving element or the optical component. In order to achieve this, an optical attenuator that attenuates an incident optical signal by a fixed attenuation amount and emits the signal is used in an optical transmission system.

In order to construct the above optical attenuator,
An optical attenuator of the type illustrated below has been considered and realized. (Example 1) In the fusion type 2 × 2 branching device, one of each two input / output ends of the optical signal is non-reflection terminated (Example 2) The radiated light emitted from the optical fiber is first In the so-called collimating system, in which the parallel rays are converged again by the second lens after being converted into parallel rays by the lens, a substance that absorbs and attenuates light is inserted between the first and second lenses (example 3) When connecting two optical fibers, the cores of the cores are shifted in the diametrical direction and connected by a method such as fusion to increase the splice loss. (Example 4) The diameters of the cores are different from each other. A method in which two different optical fibers are connected by a method such as fusion to increase the connection loss. As described in (Example 3) and (Example 4) above, by increasing the connection loss between the two optical fibers Optical attenuation that attenuates the signal Things, in the following description will be referred to as "Fiber optical attenuator".

Next, this "fiber optical attenuator" and an optical transmission system using the same will be described. The "fiber optical attenuator" shown in the above (Example 3) will be described below. FIG. 5A is a block diagram showing a configuration example of a "fiber optical attenuator" and an optical transmission system using the same. In FIG. 5A, the optical transmission system includes a semiconductor laser 501 as an example of a light source element that emits an optical signal.
An optical attenuator 502 (two shown in the figure), an optical component 503 that branches and amplifies an incident optical signal, and a light receiving element 504 that inputs the optical signal are connected by an optical fiber transmission line 505. . The optical fiber has a structure in which a core portion having a high refractive index is covered with a clad portion having a low refractive index. Therefore, the optical signal is transmitted while repeating total reflection in the core portion. Therefore, if the central axes of the cores of the two optical fibers are displaced from each other in the diametrical direction and are connected by a method such as fusion, the optical signal transmitted through the optical fiber in the former stage is incident on the optical fiber in the latter stage. Be restricted. This makes it possible to configure an optical attenuator by adjusting the amount of displacement when connecting the optical fiber of the front stage and the optical fiber of the rear stage. The optical attenuator 502 is an optical attenuator having such a configuration, and transmits the front optical fiber 5022 that transmits the optical signal incident on the light incident end 5021 and the optical signal that is incident from the front optical fiber 5022 to the optical fiber. Output end 502
And a rear optical fiber 5023 for transmitting up to four. As described above, the front optical fiber 5022 and the rear optical fiber 5023 are connected while being displaced from each other at the connection point 5025. FIG. 5B is an enlarged view of the vicinity of a connection point 5025 between the front optical fiber 5022 and the rear optical fiber 5023 in the optical attenuator 502. In FIG. 5B, the front optical fiber 5022 and the rear optical fiber 5
023 includes the core portion 5026 and the clad portion 5027, as described above. A part of the optical signal 5061 propagating through the core portion 5026 of the front optical fiber 5022 is directly incident on the core portion 5026 of the rear optical fiber 5023 and propagates through the rear optical fiber 5023 (optical signal 50
62). However, the rest of the optical signal 5061 is
It is incident on the cladding section 5027 of the rear optical fiber 5023 (see optical signal 5063). This clad part 5027
The optical signal 5063 incident on
26 is attenuated.

In the optical transmission system configured as described above, the optical signal emitted from the semiconductor laser 501 is
The optical attenuator 502 is transmitted through the optical fiber transmission line 505.
Is incident on. The optical attenuator 502 attenuates the optical signal by a predetermined amount as described above, and outputs the optical signal to the optical component 503. Optical component 5
03 has the above-mentioned function, and after branching or amplifying the optical signal into two, outputs the optical signal to the optical fiber transmission line 505 again. The optical signal transmitted through the optical fiber transmission line 505 is incident on the optical attenuator 502. The optical attenuator 502 attenuates the optical signal and outputs it to the light receiving element 504.

The above-mentioned "fiber optical attenuator" can be configured as shown in the above (Example 4). In the optical attenuator 502, the core portion 5 of the rear optical fiber 5023
The diameter of 026 corresponds to the core portion 5026 of the front optical fiber 5022.
A predetermined attenuation amount can be obtained by making the diameter smaller than the diameter (see FIG. 6A). Conversely, the diameter of the core portion 5026 of the rear optical fiber 5023 is set to the front optical fiber 50.
A predetermined amount of attenuation can also be obtained by making the diameter larger than the diameter of the core portion 5026 of 22 (see FIG. 6B).

Since this "fiber optical attenuator" has the following features, it is necessary to minimize the distortion of the optical signal transmitted by the optical transmission system, especially the reflection, and to suppress the distortion of the transmitted optical signal as much as possible. Multi
It is considered that the application to the optical transmission system using the Plexing) transmission method is effective. (1) Due to the structure of the "fiber optical attenuator", there is no optical fiber end, and therefore reflection of an optical signal does not occur. Therefore,
The transmission characteristics of the optical signal due to "multipath" (details will be described later) due to the influence of multiple reflections are not deteriorated. (2) The "fiber optical attenuator" can also connect the above-mentioned front optical fiber and rear optical fiber at the installation site of the optical transmission system. Therefore, it becomes possible to adjust to the predetermined attenuation amount at the above-mentioned laying place,
Semiconductor laser for optical transmission system (hereinafter referred to as LD)
The light output of can be easily adjusted. Therefore, L
D can be used with a bias drive current that minimizes distortion of the optical output. If an optical attenuator with a fixed attenuation other than the "fiber optical attenuator" is applied to the optical transmission system,
When adjusting the optical output of the LD as described above, a large number of optical attenuators having different attenuations are required even if a certain setting error is allowed. Moreover, if the optical attenuator with variable attenuation amount is used, the optical output adjustment of the LD as described above can be performed accurately and easily. However, an optical attenuator with variable attenuation amount has drawbacks such as high price, large size, and setting of the attenuation amount being deviated due to passage of time or vibration. Not used

By the way, in the SCM transmission method, first, a carrier wave is modulated by an electric signal to be transmitted. The light emitted from a light source element such as a semiconductor laser is directly modulated in intensity with this modulated electrical signal and converted into an optical signal.
This optical signal is transmitted in the optical fiber transmission line. This S
In an optical transmission system using the CM transmission method, distortion /
One of the causes of deterioration of optical signal transmission characteristics such as noise characteristics
One is the effect of multipath represented by multiple reflection.
The multipath will be described below by taking multiple reflection as an example.

When transmitting an optical signal by the SCM transmission method to an optical fiber transmission line having a plurality of reflection points, the light receiving element which is the receiving point of the optical signal reaches the light receiving element directly without being reflected at the reflection point. There are an optical signal (hereinafter, referred to as direct light) and an optical signal (hereinafter, referred to as delayed light) that reaches the light receiving element after being reflected at the reflection point even number of times. That is, a direct optical signal path and a delayed optical signal path are present in the path (optical path) from the light source element to the light receiving element, which is the transmission point of the optical signal. This is a phenomenon called “multipath”, which is a phenomenon that often occurs in wireless transmission systems. The delayed light that has been multiple-reflected and has reached the light receiving element has a longer path than the direct light due to the round trip of the distance between the reflection points, so that the arrival time is delayed. In addition, the reflection amount of the optical signal at each reflection point is not normally totally reflected at the reflection point (reflection amount at the open end; about -15 dB, reflection amount at a normal optical connector; about -25 to -30 dB). There is also some optical power level difference between the direct light and the delayed light.

As described above, since the direct light and the delayed light are exactly the same signal except the arrival time to the light receiving element and the optical power, they interfere with each other in the light receiving element (self-delayed homodyne). As a result, the beat of the direct light and the delayed light occurs in the frequency region of the electric signal. In the optical transmission system using the SCM transmission system, there is a problem that this beat causes noise and distortion and deteriorates the transmission characteristics. Regarding the deterioration of transmission characteristics due to this multiple reflection, the following experiments and theoretical analysis have already been reported. (1) “A. lidgard and NA Ols
son, "Generation and Canc
ellation of Second-Order
Harmonic Distortion in An
alog Optical System by In
terferometric FM-AMconvers
Ion ”, IEEE Photonic Techn
ology Letters, Vol. 2, No.
1, pp519-521, 1990 "(2)" JH Angenent, IPD Ub.
Bens, and P.M. J. de Waard, "D
ISTORION OF A MULTICARRI
ER SIGNAL DUE TO OPTICAL
REFLECTIONS. "ECOC'91, IOO
C'91, WeC8-4, 1991 "The deterioration of the transmission characteristics caused by the multipath is significantly small if not only multiple reflection but also a plurality of paths (optical paths) are physically present on the optical fiber transmission path. Must occur. In an optical transmission system, an optical signal normally propagates through the core of an optical fiber transmission line as described above. The optical signal propagating through the core portion will be referred to as "core propagation mode light" in the following description. However, if there are locations corresponding to the following (a) to (c) in the middle of the optical fiber transmission line, the core propagation mode light is incident on the clad portion of the optical fiber transmission line, and an optical signal propagating through the clad portion is generated. Will occur. The optical signal propagating through this clad is
In the following description, it will be referred to as "clad propagation mode light". (A) Part where light emitted from the light source element is collected and is incident on the optical fiber transmission line (b) Part where light emitted once from the optical fiber transmission line is incident on the optical fiber transmission line again (b) Examples of such locations include optical attenuators, optical filters, optical splitters, optical demultiplexers, etc. where optical components are inserted in the optical fiber transmission line, and optical fibers are connected using an optical connector. In this case, as an example where this optical connector does not make physical contact, such as a connection point of optical connectors where the optical connectors are not physical contacts, an optical fiber part at the end face of the optical connector generated during end face polishing as shown in FIG. There is a dent (gap) or a deviation between the center axis of curvature of the end face and the center axis of the core during PC polishing of the optical connector. (C) A place where the optical fiber transmission line is bent and wound in multiple layers with a small curvature. This clad propagation mode light is generated because a part of the core propagation mode light is incident on the clad portion. Its optical power level is smaller than that of mode light. Further, since the clad propagation mode light and the core propagation mode light have different optical paths, the arrival times at the light receiving elements are also different. Further, this clad propagation mode light also becomes an optical signal propagating in the core portion again when there is a portion corresponding to the above (a) to (c). Although this optical signal is an optical signal that propagates through the core, the arrival time at the light receiving element is always different from the arrival time of the core propagation mode light that has propagated through the core because it propagates through the cladding even temporarily. Becomes

[0011]

The fiber optical attenuator described above is excellent in applicability to an optical transmission system (especially, an optical transmission system based on the SCM transmission system) because it has the characteristics described above. However, as shown in FIG. 5B, the front optical fiber 5022 and the rear optical fiber 50
23 are connected such that the central axes of the respective core portions are displaced from each other in the diametrical direction, and the front optical fiber 502
A part of the optical signal propagating through the second core portion 5026 is radiated to the clad portion 5027 of the rear optical fiber 5023 to obtain a predetermined amount of attenuation. Therefore, the clad propagation mode light is generated. Conversely, part of the optical signal (optical signal 5064) that has propagated through the clad portion of the front optical fiber 5022 may enter the core portion 5026 of the rear optical fiber 5023 (optical signal 5065). That is, the rear optical fiber 5
023 includes a core portion 5026 and a clad portion 5027.
There will be multiple optical paths in and. The same thing can occur in the optical attenuator having the configuration shown in FIG. Since this is a phenomenon similar to the above-mentioned "multipath", when such an optical signal reaches the light receiving element, beats of the two optical signals occur in the frequency domain of the electrical signal, and the optical signal due to noise / distortion interference occurs. There is a problem that the signal transmission characteristics are deteriorated. Since the magnitude of this beat changes depending on the polarization of the propagating optical signal, it changes due to subtle differences or fluctuations such as the state of the optical fiber transmission line, changes in ambient temperature, and changes in the wavelength of the optical signal. This noise / distortion disturbance fluctuates according to the change in the size of the beat and becomes unstable in time.

Therefore, an object of the present invention is to provide an optical attenuator which prevents deterioration of transmission characteristics of an optical signal due to the influence of mutual interference between core propagation mode light and cladding propagation mode light in a light receiving element of an optical transmission system. Is to provide.

[0013]

Means for Solving the Problems and Effects of the Invention The structure of the present invention for achieving the above object will be described below.
In order to clarify the correspondence relationship with the embodiments described later, each constituent element adopted in the present invention is provided with a corresponding reference numeral. However, it should be pointed out in advance that the reference numbers are given only for easy understanding and for reference, and do not limit the scope of the claims of the present invention.

A first aspect of the present invention is an apparatus for attenuating an incident optical signal by a predetermined amount and for emitting the incident optical signal. The incident side optical fiber (102) for injecting the optical signal and the emitting side optical fiber (for emitting the optical signal ( Attenuating means (1) for increasing the connection loss and attenuating the optical signal at the connection point (105) of the incident side optical fiber and the emitting side optical fiber.
01 to 107) and a clad propagation light removing means for removing an optical signal propagating through the clad portion of the incident side optical fiber. Since the above-mentioned clad propagation light removing means removes the optical signal propagating in the clad portion of the incident side optical fiber, the optical signal propagating in the clad portion is incident on the connection point between the incident side optical fiber and the outgoing side optical fiber. Not done. Therefore, the optical signal propagating through the cladding of the incident side optical fiber is
It does not enter the core portion of the emission side optical fiber.
Therefore, at least in the optical signal incident on the light receiving element installed at the optical signal receiving point of the optical transmission system, there is one path of the optical signal propagating through the core portion.
Therefore, it is possible to reduce adverse effects on the transmission characteristics such as noise and distortion disturbance in the frequency domain of the transmission signal caused by the interference.

A second aspect of the present invention is an apparatus for attenuating an incident optical signal by a predetermined amount and for emitting the optical signal. An incident side optical fiber (102) for injecting the optical signal and an emitting side optical fiber (for emitting the optical signal ( Attenuating means (1) for increasing the connection loss and attenuating the optical signal at the connection point (105) of the incident side optical fiber and the emitting side optical fiber.
01 to 107) and a clad propagation light removing means for removing an optical signal propagating through the clad portion of the output side optical fiber. Since the clad propagation light removing means removes the optical signal propagating in the clad portion of the emitting side optical fiber, the optical signal propagating in the clad portion does not enter the emitting end of the emitting side optical fiber. Therefore, even in the optical signal incident on the light receiving element installed at the optical signal receiving point of the optical transmission system, the optical signal propagating through the clad portion and entering becomes small. Therefore, the interference with the optical signal propagating through the core portion of the optical fiber is reduced. Therefore, it is possible to reduce adverse effects on the transmission characteristics such as noise and distortion disturbance in the frequency domain of the transmission signal caused by the interference.

A third aspect of the present invention is an apparatus for attenuating an incident optical signal by a predetermined amount and emitting the optical signal, which is an incident side optical fiber (102) for injecting the optical signal and an emitting side optical fiber (for emitting the optical signal. Attenuating means (1) for increasing the connection loss and attenuating the optical signal at the connection point (105) of the incident side optical fiber and the emitting side optical fiber.
01 to 107) and a clad propagation light removing means for removing an optical signal propagating through the clad portions of the incident side optical fiber and the emitting side optical fiber. First, since the clad propagation light removing means removes the optical signal propagating in the clad portion of the incident side optical fiber, the optical signal propagating in the clad portion is incident on the connection point between the incident side optical fiber and the outgoing side optical fiber. Not done. Therefore, the optical signal propagating through the clad of the incident side optical fiber does not enter the core of the emitting side optical fiber. Further, the clad propagation light removing means also removes the optical signal propagating through the clad portion of the emission side optical fiber, so that the optical signal propagating through the clad portion is not incident on the emission end of the emission side optical fiber. Therefore, in the optical signal incident on the light receiving element installed at the optical signal receiving point of the optical transmission system, there is only one route of the optical signal propagating through the core part, and the light propagating through the clad part is incident. The signal disappears. Therefore, the optical signal incident on this light receiving element propagates through the core portion of the optical fiber transmission line in the optical transmission system by a single route. As a result, interference with other optical signals is eliminated in the light receiving element, so that adverse effects on transmission characteristics such as noise and distortion interference in the frequency region of the transmission signal can be reduced.

A fourth invention is that in any one of the first to third inventions, the central axes of the core portions of the incident side optical fiber and the outgoing side optical fiber are diametrically displaced from each other and connected. Characterize.

In a fifth aspect of the present invention, in any one of the first to third aspects, the diameter of one of the core portions of the incident side optical fiber and the emission side optical fiber is the same as that of the other core portion. It is characterized by being larger than the diameter.

In a sixth aspect based on any one of the first to fifth aspects, the clad propagating light removing means (108) has a sufficient refractive index with respect to the refractive index of the clad portion of the incident side optical fiber and the emitting side optical fiber. Liquid substances with high refractive index (1
09), and the cladding portion of the incident-side optical fiber and / or the emission-side optical fiber is characterized in that it is immersed in the liquid substance over the entire circumference of the cladding portion for a predetermined length. Since the liquid substance has a refractive index sufficiently higher than the refractive index of the cladding portions of the incident side optical fiber and the emitting side optical fiber, the cladding portion of the incident side optical fiber and / or the emitting side optical fiber. The optical signal propagating through can be attenuated and removed.

In a seventh aspect based on any one of the first to fifth aspects, the clad propagation light removing means (201) has a high refractive index with respect to the cladding portions of the incident side optical fiber and the emitting side optical fiber. It has a substance (202) having a refractive index, and the substance is attached to the clad portion of the incident side optical fiber and / or the output side optical fiber over the entire circumference of the clad portion for a predetermined length. Is characterized by. Since the above-mentioned substance has a high refractive index with respect to the refractive index of the cladding portion of the incident side optical fiber and the emitting side optical fiber, if the predetermined length is sufficiently long, the incident side optical fiber and / or the emitting side optical fiber can be used. The optical signal propagating through the cladding of the optical fiber can be attenuated and removed.

In an eighth invention according to any one of the first to fifth inventions, the clad propagation light removing means has a melting type optical component or a waveguide in the middle of the incident side optical fiber and / or the emitting side optical fiber. The optical component is inserted without using the optical connector. Since the above-mentioned fusion type optical component or waveguide type optical component has only the propagation mode of the optical signal propagating through the core of the incident side optical fiber and / or the emitting side optical fiber, the optical signal propagating through the clad is emitted. There is nothing to do. Therefore, the optical signal propagating through the cladding of the incident side optical fiber and / or the emitting side optical fiber can be attenuated and removed.

According to a ninth aspect of the present invention, in any one of the first to fifth aspects, the incident side optical fiber and the emitting side optical fiber have a core portion and a clad portion, and clad propagation light removing means ( In 301), the clad part is removed in a part of the incident side optical fiber and / or the emitting side optical fiber. As described above, when the clad portion is removed in a part of the incident side optical fiber and / or the output side optical fiber, the optical signal propagating in the clad portion is emitted to the outside in the removed portion. Since the optical signal emitted to the outside is diffused and does not enter the clad portion again, the optical signal propagating in the clad portion of the incident side optical fiber and / or the emitting side optical fiber can be removed. .

In a tenth aspect of the present invention according to any one of the first to fifth aspects, the incident side optical fiber and the emitting side optical fiber have a core portion and a clad portion, and the clad propagation light removing means is , A core part made of pure quartz, a clad part doped with a substance for lowering the refractive index of pure quartz, or a substance for lowering the refractive index of pure quartz, and an optical signal wavelength when irradiated with ionizing radiation It is characterized in that the band includes a clad part doped with a substance that absorbs light, and a sufficient dose of ionizing radiation is applied to a part or the whole of the clad propagation light removing means. When the optical fiber having the above structure is irradiated with ionizing radiation, electrons can be released in the clad portion. The liberated electrons are trapped by the lattice defects and serve as a color center. This color center absorbs light in the optical signal wavelength band. This makes it possible to remove the optical signal propagating through the cladding of the incident side optical fiber and / or the emitting side optical fiber.

An eleventh invention is any one of the first to fifth inventions, wherein the incident side optical fiber and the exit side optical fiber have a core portion and a clad portion, and the clad propagation light removing means is In the optical signal wavelength band of the input side optical fiber and / or the output side optical fiber, the clad portion is mixed with a substance that absorbs or scatters light. When an optical signal is incident on the clad portion of the optical fiber having the above-mentioned structure, the optical signal is absorbed or scattered by the action of the substance. This makes it possible to remove the optical signal propagating through the cladding of the incident side optical fiber and / or the emitting side optical fiber.

[0025]

1 is a reference diagram for explaining an optical attenuator (fiber optical attenuator) according to a first embodiment of the present invention. FIG. 1A is a diagram showing the overall configuration of the optical attenuator according to the first embodiment. In FIG. 1A, an optical attenuator transmits a front optical fiber 102 that transmits an optical signal incident on a light incident end 101 and an optical signal incident from the front optical fiber 102 to a light emitting end 103. The rear optical fiber 104 and the cladding propagation mode light remover 108
And Front optical fiber 102 and rear optical fiber 1
04 has a structure in which the core portion 106 having a high refractive index is covered with the clad portion 107 having a low refractive index. The front optical fiber 102 and the rear optical fiber 104 are connected at a connection point 1
In 05, the central axes of the core portions 106 are displaced from each other in the diametrical direction and are connected by a method such as fusion bonding. The clad propagation mode light removing part 108 contains a high refractive index liquid 109 which is a material having a sufficiently high refractive index with respect to the refractive index of the clad part 107.
Light propagating through 6 (hereinafter referred to as cladding propagation mode light) is removed. This clad propagation mode light removing section 10
8 is installed in the front optical fiber 102 and / or the rear optical fiber 104 (in FIG. 1A, the case where it is installed in the front optical fiber 102 and the rear optical fiber 104 is shown). More specifically, the front optical fiber 102 and / or the rear optical fiber 104 is dipped in the high refractive index liquid 109 around the cladding 106 for a length sufficient to remove the cladding propagation mode light. It is a thing. For example, when the refractive index of the clad 106 is about 1.46, the high-refractive-index liquid 109 is jodhed methane (refractive index n = 1.74), α-bromonaphthalene (n = 1.66), and seda oil ( n = 1.52), benzene (n = 1.50) and the like can be used.

FIG. 1B is a reference diagram for explaining a mode of propagation of an optical signal in the vicinity of the connection point 105 of the optical attenuator shown in FIG. 1A.

FIG. 1C is a vertical cross-sectional view in the vicinity of the cladding propagation mode light removing portion 108 of the optical attenuator shown in FIG. 1A, and a mode in which an optical signal propagates in this portion will be described. FIG.

The manner in which the optical signal propagates in the optical attenuator according to the first embodiment will be described below with reference to FIG. The optical signal propagated from the optical fiber transmission line (not shown) at the front stage is incident on the front optical fiber 102 via the input end 101. The optical signal incident on the front optical fiber 102 is mainly a core portion propagation mode light propagating inside the core portion 107, but a part of the optical signal is included in the cladding portion 1.
Clad propagation mode light propagating in the inside of 06. The core portion propagation mode light propagates through the core portion 107 to the connection point 105 as described above. The clad propagation mode light is almost totally reflected at the boundary between the clad 106 and the outside of the optical fiber and the boundary between the core 107 and the clad 10.
6 propagates inside. In the optical attenuator according to this embodiment,
The cladding propagation mode removing unit 108 installed in the middle of the front optical fiber 102 includes the high refractive index liquid 10 contained therein.
9 and the clad 106 have the above-described relationship of refractive index. Therefore, when the clad propagation mode light reaches the portion covered by the clad propagation mode light removing part 108, the high refractive index liquid 109 is not totally reflected at the boundary surface between the clad propagation mode removing part 108 and the clad part 106. Is transmitted to the side. Therefore, the clad propagation mode light is attenuated without being propagated through the clad portion 106 in the portion covered by the clad propagation mode light removing portion 108. Further, since the clad propagation mode light removing section 108 has a length sufficient to remove the clad propagation mode light, the clad propagation mode light disappears after the clad propagation mode light removing section 108. Therefore, the optical signal at the time of passing through the clad propagation mode light removing unit 108 installed in the front optical fiber 102 is only the core propagation mode light (see FIG. 1C).

The core propagation mode light propagates through the core portion 107 of the front optical fiber 102 and reaches the connection point 105. The optical signal has the front optical fiber 102 and the rear optical fiber 104 connected at the connection point 105 as described above. Therefore, a part of the optical signal is incident on the cladding portion 106 of the rear optical fiber 104, and the remaining optical signals are incident on the core portion 107 of the rear optical fiber 104 (see FIG. 1B). The optical signal incident on the core unit 107 is
As it is, it propagates in the core portion 107 of the rear optical fiber 104 as described above, and is emitted again from the light emitting end 103 to the optical fiber transmission line (not shown). On the other hand, the clad propagation mode light of the rear optical fiber 104 is generated by the clad 106.
Is almost totally reflected on the boundary surface between the optical fiber and the outside of the optical fiber and the boundary surface with the core portion 107, and propagates inside the clad portion 106. However, when the clad propagation mode light reaches the portion covered by the clad propagation mode light removing portion 108, it is transmitted to the high refractive index liquid 109 side as described above. Therefore, the clad propagation mode light is attenuated without propagating in the clad portion 106. Therefore, the cladding propagation mode light disappears again after the cladding propagation mode light removing unit 108. Therefore, since only the core propagation mode light is incident on the light receiving element (not shown) or the like installed in the subsequent stage of the optical attenuator, no beat is generated.

In the above-described embodiment, the clad propagation mode light removing section 108 immerses the front optical fiber 102 and the rear optical fiber 104 for a sufficient length, and completely removes the clad propagation mode light. It has such a structure. However, even if the immersed portion is short, the effect of removing the clad propagation mode light is sufficient.

The optical attenuator in which the core diameter of one of the two optical fibers in the connecting portion is smaller than the core diameter of the other so as to increase the connection loss and obtain the required attenuation amount can also be used. The action is exactly the same.

FIG. 2 is a reference diagram for explaining an optical attenuator (fiber optical attenuator) according to a second embodiment of the present invention. FIG. 2A is a diagram showing the overall configuration of the optical attenuator according to the second embodiment. In FIG. 2A, the optical attenuator includes a light incident end 101, a front optical fiber 102, a light emitting end 103, a rear optical fiber 104, and a cladding propagation mode light elimination coat 201. Note that FIG.
In (a), the same components as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. The clad propagation mode light removing coat 201 includes a coating material 202 which is a material having a higher refractive index than the refractive index of the clad portion 107, and removes light propagating through the clad portion 106.
The cladding propagation mode light removal coat 201 is provided on the front optical fiber 102 and / or the rear optical fiber 104. More specifically, the front optical fiber 10
2 and / or the back optical fiber 104 has the coating material 202 deposited around the cladding 106 for a length sufficient to remove the cladding propagation mode light.

FIG. 2B is a vertical cross-sectional view of the vicinity of the cladding propagation mode light removal coat 201 of the optical attenuator shown in FIG. 2B, and the manner in which an optical signal propagates in this portion will be described. FIG.

Hereinafter, the manner in which the optical signal propagates in the optical attenuator according to the second embodiment will be described with reference to FIG. This is basically the same as the mode in which the optical signal propagates in the optical attenuator according to the first embodiment. That is, the clad propagation mode light of the front optical fiber 102 is transmitted to the coat material 202 side by the coat material 202 of the clad propagation mode removing coat 201 installed in the middle of the front optical fiber 102 and emitted. Therefore, the clad propagation mode light is not propagated through the clad portion 106 in the portion to which the coating material 202 is attached and is attenuated. Therefore, the optical signal at the time of passing through the cladding propagation mode light elimination coat 201 installed in the front optical fiber 102 is only the core propagation mode light.

The optical signal propagating through the rear optical fiber 104 is connected to the front optical fiber 102 and the rear optical fiber 104 as described above.
4 is incident on the core portion 107 and the cladding portion 106. The optical signal incident on the core portion 107 propagates as it is in the core portion 107 of the rear optical fiber 104, and is emitted again from the light emitting end 103 to the optical fiber transmission line (not shown).

On the other hand, the cladding propagation mode light of the rear optical fiber 104 is transmitted to the coating material 202 side by the coating material 202 of the cladding propagation mode removing coat 201 and is emitted. Therefore, the clad propagation mode light is
In the portion where the coating material 202 is attached, the cladding portion 106 is not propagated and is attenuated. Therefore, the optical signal emitted from the light emitting end 103 is only the core propagation mode light. Therefore, since only the core propagation mode light is incident on the light receiving element (not shown) or the like installed in the subsequent stage of the optical attenuator, no beat is generated.

As an optical fiber used in a conventional optical attenuator, a material having a refractive index higher than that of the clad portion may be used for a buffer layer (coating layer) around the clad portion.
However, the material used for the buffer layer has the following requirements (a) to (d). (A) Strong to protect the optical fiber from external pressure and friction (b) Soft so that loss does not increase due to external force (c) Close contact with the optical fiber and no slippage (Easy removal) d) Chemically stable Therefore, it is difficult to select the material forming the buffer layer only because it has a high refractive index, and the refractive index is slightly higher than that of the clad portion. It was Therefore, when this type of optical fiber is used as the optical fiber of the optical attenuator, a sufficient length cannot be ensured for reasons such as storage in the device. for that reason,
The removal of the clad propagation mode light is insufficient, and it is inappropriate to use the conventional optical attenuator in the optical transmission system using the SCM transmission method.

In the present embodiment, even if the difference in the refractive index between the cladding portion 106 and the coating material 202 is small, the length of the cladding propagation mode light removing coat 201 is set to a length sufficient to remove the cladding propagation mode light. Therefore, a sufficient effect of removing the cladding propagation mode light can be obtained as in the first embodiment. In an experiment conducted by the inventor of the present application, when an optical fiber of a certain manufacturer is used as a pigtail, if the length from the light incident end and the light emitting end to the connection point is several meters, the clad propagation mode light transmission characteristic is obtained. Was eliminated, but with other manufacturers' optical fibers, 10
It was not possible to completely eliminate the effect even if the length was over m. As described above, when a coating material having a small difference in refractive index is used, the above length is required even in an optical transmission system using the SCM transmission method. Therefore, in this embodiment, if the length of the optical fiber is increased, the optical fiber must be bent for storage reasons. In this bent portion, cladding propagation mode light may be generated depending on the curvature or the like, so that it is more effective to select the coating material 202 having a high refractive index and shorten the optical fiber length.

Next explained is an optical attenuator according to the fourth embodiment of the invention. This optical attenuator is replaced with the clad propagation mode light removing section 108 of the optical attenuator shown in FIG.
Front optical fiber 102 and / or rear optical fiber 1
In the middle of 04, a fusion type optical component or a waveguide type optical component is inserted by a connecting method such as fusion bonding without using an optical connector or the like. The fused optical component is an optical component in which two or more optical fibers are twisted and stretched while being melted at a certain place, and usually used as an optical branching device / optical demultiplexer. The waveguide type optical component is an optical component in which an optical waveguide is formed on a substrate and light propagates through the optical waveguide. Not only is it used as a passive component such as an optical branching device, but also an external modulator, etc. It is also used as an active optical component.
The fusion type optical component or the waveguide type optical component propagates only an optical signal having a predetermined propagation mode. That is, the above optical component can directly emit the core propagation mode light that has entered from the core portion 107 of the front optical fiber 102 and / or the rear optical fiber 104, and can propagate the light to the connection point 105 or the light emitting end 103. . However, the clad propagation mode light that has entered from the clad portion 106 of the front optical fiber 102 and / or the rear optical fiber 104 does not have a propagation mode because it is not considered to be propagated, and thus is emitted from the above optical component. Therefore, it does not propagate to the connection point 105 or the light emitting end 103.

The operation of the optical attenuator according to the fourth embodiment configured as described above is basically the same as that of the conventional optical attenuator, but this embodiment is different from the first embodiment. Similarly, the cladding propagation mode light generated before these optical components is removed,
The connection point 105 prevents the optical fiber from reaching the output end of the optical fiber. That is, as described above, by inserting the optical component in the middle of the optical fiber, the passage of the clad propagation mode light is blocked, and only the core propagation mode light is transmitted to the optical fiber emitting section, thereby preventing the transmission deterioration.

The reason for inserting the optical component by fusion or the like without using the optical connector in this embodiment is that the clad propagation mode light may be generated again when the optical connector is used.

FIG. 3 is a diagram showing the overall structure of an optical attenuator (fiber optical attenuator) according to a third embodiment of the present invention.
In FIG. 3, the optical attenuator includes a light incident end 101, a front optical fiber 102, a light emitting end 103, a rear optical fiber 104, and a clad propagation mode light blocking unit 301. In FIG. 3, the same components as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. The clad propagation mode light blocking unit 301 is, like the clad propagation mode removing unit 108 of the first embodiment, intended to prevent clad propagation mode light from propagating after the cladding propagation mode light blocking unit 301. More specifically, the clad portion 106 at a part of the front optical fiber 102 and / or the rear optical fiber 104 is removed, and the core portion 107 is exposed to the outside. A mode in which an optical signal propagates in the optical attenuator according to the third embodiment will be described. this is,
This is basically the same as the mode in which the optical signal propagates in the optical attenuator according to the first embodiment. That is, the clad propagation mode light incident on the front optical fiber 102 is
The light is emitted to the clad propagation mode light blocking unit 301. In the clad propagation mode light blocker 301, the clad propagation mode light is diffused, so that it does not enter the cladding part 106 again and does not reach the connection point 105. Also,
The clad propagation mode light that has entered the rear optical fiber 104 is also diffused in the clad propagation mode light blocker 301 installed in the rear optical fiber 104 and therefore does not reach the light emitting end 103. Therefore, the light emitting end 10
The optical signal emitted from 3 is only core propagation mode light. Therefore, since only the core propagation mode light is incident on the light receiving element (not shown) or the like installed in the subsequent stage of the optical attenuator, no beat is generated.

FIG. 4 is a reference diagram for explaining an optical attenuator (fiber optical attenuator) according to a fifth embodiment of the present invention. FIG. 4A is a diagram showing the overall configuration of the optical attenuator according to the fifth embodiment. In FIG. 4A, an optical attenuator transmits a front optical fiber 102 that transmits an optical signal incident on a light incident end 101 and an optical signal incident from the front optical fiber 102 to a light emitting end 103. And a rear optical fiber 104 for The front optical fiber 102 and the rear optical fiber 104 are connected at a connection point 105 by a method such as fusion in which the central axes of the core portions 106 are displaced from each other in the diametrical direction. Furthermore, the core portion 1 of the front optical fiber 102 and the rear optical fiber 104
07 is made of pure quartz, and the clad portion 106 thereof is made of pure quartz doped with a substance such as fluorine that lowers the refractive index. In addition, the cladding portions 106 of the front optical fiber 102 and the rear optical fiber 104 are substances that lower the refractive index and substances that absorb light in the optical signal wavelength band when irradiated with ionizing radiation (for example, boron or phosphorus). Pure quartz may be doped. The front optical fiber 1 thus configured
02 and the rear optical fiber 104 in whole or part
Irradiate ionizing radiation such as rays.

By the way, when the optical fiber is irradiated with ionizing radiation, electrons are released. The liberated electrons are trapped by the lattice defects in the clad and become a "color center".
This color center absorbs light of wavelengths from visible light to near infrared light according to its energy rank. When the optical fiber material is pure quartz, during irradiation of ionizing radiation,
The resulting color center absorbs light. However, after irradiation of ionizing radiation, the color center disappears immediately, and the optical fiber is restored to its original state. However, when the material of the optical fiber is pure quartz doped with a substance such as fluorine or germanium for the purpose of raising or lowering the refractive index, the effect of light absorption by the color center during irradiation of ionizing radiation is It is larger than the case. Further, even after irradiation with ionizing radiation, the color center does not disappear, and the state before irradiation is hardly restored, so that the effect of light absorption is maintained. Here, the amount of light absorption increases in proportion to the total dose of irradiated ionizing radiation.

The operation of the optical attenuator according to the fifth embodiment having the above-mentioned configuration is basically the same as that of the optical attenuator described in the section "Prior Art". However, as described in the first embodiment and the like, the clad propagation mode light generated in the middle of the optical fiber transmission line or at the connection point of the optical fibers is absorbed by the color center, so that the light emitting end 103 It is possible to prevent the clad propagation mode light from reaching. On the other hand, in the core propagation mode light, since the core portion is made of pure quartz and the color center disappears as described above, absorption or the like does not occur, and the connection point 105 and the light emitting end 103 are not generated.
To reach. Therefore, since only the core propagation mode light is incident on the light receiving element (not shown) or the like installed in the subsequent stage of the optical attenuator, no beat is generated.

In the optical attenuator according to this embodiment, if the total dose of ionizing radiation applied to the front optical fiber 102 and the rear optical fiber 104 is large, the clad propagation mode light is removed even if the length of these optical fibers is small. Has the effect of
Conversely, irradiating these optical fibers with ionizing radiation for a sufficient length has the effect of removing the cladding propagation mode light even if the total dose is small.

In the optical attenuator according to this embodiment,
The effect of the present invention can be further enhanced by irradiating the ionizing radiation as close as possible to the connection point 105 of the front optical fiber 102 and the rear optical fiber 104.

Next explained is an optical attenuator according to the sixth embodiment of the invention. This optical attenuator has the same configuration as that of the conventional optical attenuator shown in FIG. 5, except that the cladding portion of the front optical fiber and / or the rear optical fiber absorbs or scatters the optical signal in the optical signal wavelength band. This is different from the conventional optical attenuator shown in FIG. 5 in that OH groups and minute bubbles that are attenuated are mixed.

The operation of the optical attenuator according to the sixth embodiment having the above-mentioned configuration is basically the same as that of the optical attenuator described in the section "Prior Art". However, as described in the first embodiment and the like, the clad propagation mode light generated in the middle of the optical fiber transmission line or at the connection point of the optical fibers is mixed in the clad portion.
Since it is absorbed or scattered by the H group and minute bubbles, it is possible to prevent the clad propagation mode light from reaching the light emitting end. On the other hand, the core propagation mode light is not absorbed because the core is the same as an ordinary optical fiber, and the connection point 105 and the light emitting end 10 are not generated.
Reach 3 Therefore, since only the core propagation mode light is incident on the light receiving element (not shown) or the like installed in the subsequent stage of the optical attenuator, no beat is generated.

Incidentally, the cladding propagation mode light removing section 108 in the first embodiment, the cladding propagation mode light removing coat 201 in the second embodiment, the portion irradiated with ionizing radiation in the fifth embodiment, and the sixth embodiment. The portion in which the substance is mixed in the embodiment may be provided at a portion of the front optical fiber 102 and / or the rear optical fiber 104 away from the connection point 105 as described above, but may be provided from the front optical fiber 102 to the rear portion. Connection point 1 across optical fiber 104
Even if it is provided in a mode including 05, the above-mentioned effects do not change.

[Brief description of the drawings]

FIG. 1 is a reference diagram for explaining an optical attenuator according to a first embodiment of the present invention.

FIG. 2 is a reference diagram for explaining an optical attenuator according to a second embodiment of the present invention.

FIG. 3 is a diagram showing an overall configuration of an optical attenuator according to a third embodiment of the present invention.

FIG. 4 is a reference diagram for explaining an optical attenuator according to a fifth embodiment of the present invention.

FIG. 5 is a reference diagram for explaining a conventional optical attenuator.

FIG. 6 is a cross-sectional view of a conventional optical attenuator, in which two optical fibers having different core diameters are connected by a method such as fusion to increase the connection loss, and a cross-section near the connection point is schematically illustrated. It is a figure shown and demonstrated.

FIG. 7 is a diagram showing a state of optical connector joining with a gap that may be used in a normal optical transmission system.

[Explanation of symbols]

 101 ... Light incident end 102 ... Front optical fiber 103 ... Light emitting end 104 ... Rear optical fiber 105 ... Connection point 106 ... Clad part 107 ... Core part 108 ... Clad propagation mode light removing part 109 ... High refractive index liquid 201 ... Clad Propagation mode light removal coat 202 ... Coating material 301 ... Clad propagation mode light blocking unit

Claims (11)

[Claims] 1. An apparatus for attenuating an incident optical signal by a predetermined amount and outputting the attenuated optical signal, the apparatus including an incident side optical fiber for injecting the optical signal and an emitting side optical fiber for emitting the optical signal. At the connection point between the optical fiber and the outgoing side optical fiber, an attenuating means for increasing the connection loss and attenuating the optical signal, and a clad propagation light removing means for removing the optical signal propagating through the clad portion of the incident side optical fiber are provided. , Optical attenuator. 2. A device for attenuating an incident optical signal by a predetermined amount and outputting the attenuated optical signal, the device including an incident side optical fiber for injecting the optical signal and an emitting side optical fiber for emitting the optical signal. At the connection point between the optical fiber and the outgoing side optical fiber, an attenuating means for increasing the connection loss and attenuating the optical signal, and a clad propagation light removing means for removing the optical signal propagating through the clad portion of the outgoing side optical fiber are provided. , Optical attenuator. 3. An apparatus for attenuating an incident optical signal by a predetermined amount and outputting the attenuated optical signal, which includes an incident side optical fiber for injecting the optical signal and an emitting side optical fiber for emitting the optical signal. Attenuating means for increasing the connection loss and attenuating the optical signal at the connection point of the optical fiber and the outgoing side optical fiber, and clad propagation for removing the optical signal propagating through the clad part of the incident side optical fiber and the outgoing side optical fiber. A light attenuator, comprising: a light removing means. 4. The optical fiber according to claim 1, wherein central axes of core portions of the incident side optical fiber and the emitting side optical fiber are displaced from each other in a diametrical direction and are connected to each other. Light attenuator. 5. The diameter of one of the core portions of the incident-side optical fiber and the emission-side optical fiber is larger than the diameter of the other core portion.
The optical attenuator according to claim 1. 6. The clad propagation light removing means has a liquid substance having a refractive index sufficiently higher than the refractive indexes of the cladding parts of the incident side optical fiber and the emitting side optical fiber, The side optical fiber and / or the clad portion of the output side optical fiber is immersed in the liquid substance over the entire circumference of the clad portion for a predetermined length, any one of claims 1 to 5 characterized in that An optical attenuator as described in. 7. The clad propagation light removing means comprises a substance having a high refractive index with respect to the refractive indexes of the cladding parts of the incident side optical fiber and the emitting side optical fiber, and the incident side optical fiber. The material is adhered to the clad part of the output side optical fiber over the entire circumference of the clad part for a predetermined length, and / or the clad part of the outgoing side optical fiber according to any one of claims 1 to 5. Optical attenuator. 8. The clad propagation light removing means inserts a fusion type optical component or a waveguide type optical component in the middle of the incident side optical fiber and / or the emission side optical fiber without using an optical connector. The optical attenuator according to any one of claims 1 to 5. 9. The entrance-side optical fiber and the exit-side optical fiber have a core portion and a clad portion, and the clad propagation light removing means includes the entrance-side optical fiber and / or the exit-side optical fiber. The optical attenuator according to any one of claims 1 to 5, wherein the cladding is removed from a part of the fiber. 10. The entrance-side optical fiber and the exit-side optical fiber have a core portion and a clad portion, and the clad propagation light removing means includes a core portion made of pure quartz and refraction to pure quartz. Clad part doped with a substance for decreasing the refractive index, or a clad part doped with a substance for decreasing the refractive index of pure quartz and a substance that absorbs light in the optical signal wavelength band when irradiated with ionizing radiation. The optical attenuator according to any one of claims 1 to 5, characterized in that a part or the whole of the clad propagation light removing means is irradiated with a sufficient dose of ionizing radiation. 11. The entrance-side optical fiber and the exit-side optical fiber have a core portion and a clad portion, and the clad propagation light removing means includes the entrance-side optical fiber and / or the exit-side optical fiber. The optical attenuator according to claim 1, wherein the optical attenuator is a clad portion mixed with a substance that absorbs or scatters light in a fiber optical signal wavelength band.